1. Field of the Invention
The invention relates to a thruster that uses microwaves, from a ground-based or on-board source, to provide energy for propulsion.
2. Description of the Prior Art
Wireless power transmission within and outside the atmosphere is reviewed in the article by William C. Brown; Beamed Microwave Power And Its Application To Space, IEEE Transactions on Microwave Theory and Techniques Vol. 40 No. 6 (1992). The high power microwave aspect is detailed by J. Benford and J. Swegle; High-Power Microwaves. Artech House (1992).
The propulsion system generates thrust using microwave energy-addition to a propellant via a heat exchanger. Heat exchangers are discussed generally by O. Levenspiel; Engineering Flow and Heat Exchange, and in the context of nuclear rockets by Turchi; Propulsion Techniques, Action and Reaction. AIAA (1998). Nuclear thermal rockets, simple ascent trajectory modeling, and methods of thermodynamic nozzle design, are presented in the classic text by G. P. Sutton; Rocket Propulsion Elements. For the case of airbreathing propulsion, this open-cycle thermodynamic process is described by Mattingly; Elements of Gas Turbine Propulsion. The design of ground based microwave energy sources for rocket propulsion is described in J. Benford et. al., “Space Propulsion and Power Beaming Using Millimeter Systems,” in H. Brandt ed., Intense Microwave Pulses III, SPIE 2557, 179 (1995); and in Benford et. al. “Propulsion of Small Launch Vehicles Using High Power Millimeter Waves,” Intense Microwave Pulses II, SPIE 2154, 198 (1994).
High power microwave breakdown of gases, for example in H2 and air, can be a limiting factor in some aspects of design. Breakdown of this kind is explained by J. M. Meek & J. D. Craggs; Electrical Breakdown of Gases.
A recent overview of the present art in microwave-material interactions for heating, including references, is given by the National Research Council; Microwave Processing of Materials. The field of microwave materials processing shares common disciplines with microwave heat-exchange thruster design. In particular, it shares absorption physics and experimental, analytical, and computational techniques for the design of microwave absorbing structures. The microwave absorbing structure itself will necessarily be constructed of refractory materials, which are treated in an engineering context by H. O. Pierson; Handbook of Refractory Carbides and Nitrides: Properties, Characteristics, Processing and Applications (1996).
Some types of microwave and laser propelled rocket engines have been proposed in the past. In these engines, a high power microwave or laser beam is directed at the launch vehicle to be captured and focused onto a suitable working fluid, such as hydrogen. The working fluid is thereby heated to very high temperatures and expelled from a conventional rocket nozzle with exhaust velocities significantly higher than those obtainable with chemical rocket engines. (See for example, the papers: “Microwave Rocket Concept,” International Astronautical Congress, Vol. 16, Athens, 1965, pp. 175–199 by J. L. Schad and J. J. Moriarty; and “NASA's Laser Propulsion Project,” Astronautics & Aeronautics, September 1982, pp. 66–73 by L. W. Jones and D. R. Keefer.) Unfortunately, all of these attempts at circumventing the initial mass problem have failed by a wide margin. Thus, it was subsequently believed that reusable single stage to orbit (SSTO) vehicles propelled by chemical rocket engines would be the most economical transportation system for launching manned vehicles to low Earth orbit (See, “The Future Space Transportation Systems Study,” Astronautics & Aeronautics, June 1983.). At present, the failed development of Venture Star, an SSTO launcher, has led to renewed interest in two stage to orbit (TSTO) vehicles. (See for example, “Integrated Space Transportation Plan”, NASA, 2002).
Although microwave and laser propelled launch vehicles were found to be technically heretofore impractical for launching manned space vehicles, they had one common and very important characteristic: the energy generating mechanism used to accelerate the vehicle is located off the vehicle. Thus, the amount of energy that can be used to accelerate the vehicle is limited only by the physics of energy transmission from source to vehicle. Moreover, since the energy source is physically removed from the vehicle, the vehicle is not burdened by having to accelerate its inertial mass. In principle, the combination of these two operating characteristics has the potential of giving a “telepropelled” vehicle of very high performance.
Minovitch, “Electromagnetic Transportation System for Manned Space Travel,” U.S. Pat. No. 4,795,113 (1989) describes an exotic electromagnetically propelled space transportation system in which the launch vehicle is equipped with a plurality of superconducting propulsion coils extending along the fuselage and is accelerated to orbital velocities inside a vacuum tube by a 1,530 km long electromagnetic linear accelerator. The vacuum tube is evacuated by utilizing the accelerator as a giant vacuum pump wherein a free-moving, magnetically propelled, air-tight piston is driven through the entire tube at low speed thereby forcing the air directly out the end. The accelerator is embedded deep underground with a maximum depth of 46 km and emerges near the summit of a high mountain. The system is powered by the Earth's gravitational field whereby natural hydro and geothermal energy is converted into electrical energy.
Besides having a conventional chemical rocket engine, the Minovitch space propulsion system includes six high power electron cyclotron resonance plasma engines mounted around the central rocket nozzle. Each of these electromagnetic engines are 1.5 m (4.92 ft) in diameter and generate an effective propulsive power of 5 MW. The plasma engines use argon or nitrogen propellant. The electric power source used to operate the plasma engines is derived from the thousands of high field superconducting vehicle propulsion coils and superconducting magnetic shielding coils mounted inside the vehicle's pressure hull. Thus, the propulsion coils are not only used to launch the vehicle from Earth by magnetic forces between the drive coils of the electromagnetic accelerator but also as inductive energy storage systems for operating the vehicle's plasma accelerator engines, for auxiliary space propulsion as well. The DC electric current is extracted from the superconducting coils via the electrical systems and fed into high efficiency Magnetron or Amplitron microwave generators located in the rear section of the vehicle. The microwaves are fed into a system of high power waveguides leading into the plasma accelerators. The detailed design, construction and operating principles of these high power engines (using superconducting drive coils) can be found in “Solar Powered, Self-Refueling, Microwave Propelled Interorbital Transportation System,” AIAA 18th Thermophysics Conference, Jun. 1–3, 1983, Montreal, Canada, AIAA paper No. 83–1446 by M. A. Minovitch.